1. Field of the Invention
The present invention relates to an improved method of cleaning a nuclear steam generator and other tube bundle heat exchangers by removing the buildup of sedimentation and other deposits which accumulate on the heat exchanger tubes, on the tube support plates at various elevations, and on other surfaces of the heat exchanger vessel through utilization of a repetitive shock wave induced in a liquid medium placed in the heat exchanger vessel. The repetitive shock wave serves to effectively and safely loosen the products of corrosion and other elements and thereby facilitates their easy removal through flushing and vacuuming the vessel.
2. Description of the Prior Art
One of the major components in a power generating facility such as a nuclear power plant is the steam generator or heat exchanger portion of the facility. Large scale heat exchanger systems are essentially comprised of a primary system which contains a large number of individual tubes which have fluid circulating through them and a secondary system which consists of a second fluid surrounding said tubes contained within a housing which enwraps both systems. Heat is transferred from the fluid running through these heat exchanger tubes to the fluid in the secondary system which is itself eventually turned to steam. The steam, in turn, generates power.
These heat exchangers or steam generators have experienced many problems due to the buildup of products of corrosion, oxidation, sedimentation and comparable chemical reactions within the heat exchanger. The problem of magnetite buildup at the junctions of the primary heat exchanger tubes and the support plates for those tubes, and further magnetite buildup within the crevices between the tubes and their support plates was extensively treated in U.S. Pat. No. 4,320,528. That patent addressed the use of ultrasonic methods to facilitate the removal of the magnetite from those junctions.
At the bottom of the heat exchanger vessel is a tube sheet. This thick metal plate which acts as the support base for numerous heat exchanger tubes is a primary support structure in the steam generator. In addition to the problems of magnetite buildup at the junctions and inside the crevices of the primary heat exchanger tubes and their support pltes, a second problem has also troubled steam generators for many years. There is a buildup of sedimentation of "sludge" which accumulates in the bottom of heat exchanger vessels. This sludge includes copper oxide, magnetite and other oxidation or corrosion products which have not adhered to the tubing or other surfaces and therefore accumulate at the bottom. The sludge pile rests on top of the tube sheet and may form a thick layer. The sludge further accumulates in the crevices between the tube sheet and the primary heat exchanger tubes which are embedded in the tube sheet for support. The problem of removing the sludge which enters the deep crevices in the tube sheet was addressed in presently pending patent application Ser. No. 06/370,826 filed on 4/22/82. Patent application Ser. No. 06/370,826 solves the problem of removing sludge from the deep crevices through use of specilaized ultrasonic waves which are directed in a certain way to produce the desired result. A method of removing the sludge on the lowermost tube support sheet through the use of pressure pulses was addressed in presently pending U.S. patent application Ser. No. 06/486,352 filed 4/19/83.
In addition to adhering on the tube support sheet, the sludge and other deposits also adhere to the interior of the heat exchanger tubes. A method of pressure pulse cleaning and removing sludge and other deposits from the interior of heat exchanger tubes is addressed in presently pending U.S. patent application Ser. No. 06/604,048 filed 4/26/84.
In addition to the above problems which have been addressed by the above referenced patent and patent applications, corrosion byproducts deposit on the exterior surfaces of heat exchanger tubes and on the tube support plates as well as on the interior sides of the heat exchanger vessel. These deposits, which are commonly found in the upper region of the steam generator, can restrict the water flow in the heat exchange process and also accelerate corrosion of the tube support plates, the heat exchanger, and the metal walls of the heat exchanger vessel.
The buildup of sludge on the tube support plates and the heat exchanger tubes degrades the heat transfer process from the fluid in the primary system to the fluid in the secondary system, and may also restrict secondary fluid flow. The heat exchanger tubes can also be damaged. As a result, it is very important to clean the heat exchanger or steam generator to effectively remove the sludge from the surface of the tube support plates, the heat exchanger tubes, and other surfaces such as the walls of the heat exchanger vessel. Much of the prior art referenced in the previous patent and patent applications employs the use of ultrasonics. While the methods discussed are effective and valuable, the use of ultrasonics has several disadvantages. First, in order to generate the ultrasonic waves, expensive transducers must be used. This requires considerable effort and expense to bring the ultrasonic transducers to the site of the steam generator and then putting them in their proper place at the location of or within the steam generator. Second, in order to achieve an effective level of ultrasonic waves, it is often necessary to cut away a portion of the steam generator wall and put the face of the transducer at the location of the cut away portion. Many owners of the power plants which incorporate a steam generator are reluctant to have a portion of a wall cut away and then later welded back in place after the steam generator has been cleaned.
A third problem which arises with prior art applications is the use of corrosive chemicals to assist in the cleaning operation. While the chemicals serve to clean and remove the sludge, they also serve to eat away at the various components of the steam generator. Therefore, it is desirable to find a method of cleaning which does not require the use of corrosive chemicals. One method known in the prior art is called water lancing. This is in effect the use of a jet of water which is shot into the sludge pile for the purpose of loosening the sludge. The results so far have not been very encouraging. The loosening process is not very effective and in addition there may be a problem of using the jet of water to impinge against the heat exchanger tubes at that location. The jet of water might cause sludge particles to reflect onto and then off the heat exchanger tubes, thereby possibly resulting in damage to these tubes. In addition, the technique of water lancing is not useful for removing sludge and deposits from the tube support plates, tubes and other surfaces above the tube sheet because the access to these regions is very limited. Also in many steam generator designs there is not even sufficient access to utilize water lancing on the bottom tube sheet. The close crowding of a large multiplicity of tubes and the high elevation make this method ineffective.
Therefore, although the use of ultrasonics combined with chemicals and the use of a jet of water are all known in the prior art for cleaning and removing sludge at the bottom of a heat exchanger or steam generator, none of these methods can be employed without the significant problems discussed above. The methods are also not effective in the upper regions of the steam generator due to the restricted available space.
Methods of pressure pulse cleaning have been addressed for cleaning the tube support plate and for cleaning the interior of heat exchanger tubes but no effective method has been previously discussed for cleaning the tube support plates, the exterior surfaces of heat exchanger tubes, and other heat exchanger surfaces such as the walls of the heat exchanger vessel without the use of ultrasonics and corrosive chemicals. Pressure pulse cleaning has not been discussed for removing sludge from these additional and critical areas of the steam generator vessel.